Directed evolution of natural microbial populations has been shown to be an efficient approach to study molecular mechanisms of natural selection, adaptation and speciation. The short generation time, large population size, simple life cycle, and ease of maintenance and storage make bacterial and viral systems exceedingly suitable for evolution experiments. Nevertheless, such experiments typically require multiple iterations of the mutation-selection cycle that implies (1) diversification of parental genetic material by spontaneous or induced mutagenesis, and (2) selective amplification of successful genotypes through differential reproduction of the microorganisms under defined environmental constraints. At the end of the procedure, the acquired genetic changes can be examined and related to those phenotypic features, which differentiate the evolved cell lineages from the ancestral strain.
A simple way to direct the evolution of a microbial population is to make it propagate under an appropriately applied stress. Stress-induced imbalances in cellular metabolism result in reduced fitness of the wild type lineage. At the same time, some of the emerging mutants may exhibit a substantial tolerance of the harmful factor. During prolonged cultivation under stressful conditions, these resistant phenotypes will gradually substitute the wild type. Accordingly, the population will drift towards higher frequencies of the mutated genes associated with the resistant clones. The original genotype will eventually be replaced with a new one, which confers an improved fitness on the microbes exposed to the hostile environment.